Method of dehumidifying an indoor space using outdoor air

ABSTRACT

A method for dehumidifying air in an indoor space using outside air is comprised of the steps of: sensing whether there is a demand for dehumidification in the space; in response to a demand for dehumidification, determining whether outdoor air is suitable for dehumidification of the space; if the outdoor air is determined to be suitable, admitting outdoor air into the air supplied to the space; and selectively temperature conditioning the supply air in accordance with a predetermined temperature condition therefor. The method is disabled in response to a demand for heating or sensible cooling in the space. Further, the ratio of outdoor air to return air in the supply air is kept relatively constant, irrespective of whether the predetermined temperature condition is satisfied.

TECHNICAL FIELD

This invention relates generally to space conditioning systems forconditioning the temperature and humidity of an indoor space and inparticular to a method of dehumidifying the space using outdoor air.

BACKGROUND ART

Central air conditioning systems, including systems that use a vaporcompression refrigerant to cool air circulated to an indoor space, oftenrequire relatively accurate control of humidity in the air within thespace. In one such system, as described in U.S. Pat. No. 6,644,049, areheat heat exchanger is positioned downstream of the system evaporatorto reheat air that is cooled and dehumidified by the evaporator.Therefore, dehumidified air can be delivered to the space withoutappreciably cooling the space.

In the system described in U.S. Pat. No. 6,644,049, outdoor air is notadmitted into the system in response to a demand for dehumidification inthe space. In the air conditioning system described in U.S. Pat. No.6,427,461, outdoor air is admitted into the supply air stream inresponse to a demand for dehumidification if the enthalpy of the outdoorair is suitable. Typically, if the enthalpy of the outdoor air issuitable for dehumidification, it is cooler than the desired temperatureof the space. If as a result of introducing cooler outdoor air into thesupply air stream, the supply air temperature drops below apredetermined setpoint, the amount of outdoor air admitted into thesystem is reduced to try to prevent overcooling of the space. However,there is no provision to heat the supply air in the absence of a demandfor heating in the space.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided fordehumidifying an indoor space using outdoor air. The method is comprisedof the steps of: sensing whether there is a demand for dehumidificationin the space; in response to a demand for dehumidification, determiningwhether outdoor air is suitable for dehumidification of the space; ifthe outdoor air is determined to be suitable, admitting outdoor air intothe space; and selectively temperature conditioning the outdoor airprior to admitting it into the space in accordance with a predeterminedtemperature condition for air supplied to the space.

In accordance with one embodiment of the invention, the method includesdetermining whether the temperature of the outdoor air satisfies apredetermined outdoor air temperature condition.

In accordance with another embodiment of the invention, the methodincludes determining whether the humidity of the outdoor air satisfies apredetermined outdoor air humidity condition.

In accordance with still another embodiment of the invention, the methodincludes sensing the temperature of the air supplied to the space andheating the air supplied to the space as needed to maintain thepredetermined temperature condition for the supply air, even in theabsence of a demand for heating in the space.

In accordance with yet another embodiment of the invention, the methodincludes sensing whether there is a demand for temperature conditioningof the air in the space. In response to a demand for temperatureconditioning of the air in the space, the dehumidifying method isdisabled.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic showing the circulation of air in an airconditioning system;

FIG. 2 is a block diagram showing the control for the air conditioningsystem of FIG. 1; and

FIG. 3 is a flow diagram depicting the dehumidification operation inaccordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described withreference to the accompanying drawings. Like parts are marked in thespecification and drawings with the same respective reference numbers.In some instances, proportions may have been exaggerated in order todepict certain features of the invention.

Referring now to FIG. 1, there is illustrated an air conditioning systemin accordance with an embodiment of the present invention and generallydesignated by numeral 10. Air conditioning system 10 is shown, by way ofexample, as a so-called “rooftop” system having a generally rectangularbox-like cabinet 12 adapted to be mounted on a generally horizontalsurface such as a rooftop 14. Air conditioning system 10 is adapted todeliver conditioned air to an enclosed space 16 by way of an opening 18and air within the enclosed space 16 is returned to system 10 by way ofan opening 19 to cabinet 12 for conditioning by system 10.

System 10 preferably includes a vapor compression refrigeration unit 20,including a compressor (not shown) for circulating a conventionalrefrigerant fluid between indoor and outdoor heat exchanger coils.Indoor heat exchanger 32 is disposed within cabinet 12 and is arrangedin the path of the air flow through cabinet 12, as shown by the arrows.When system 10 is operated in a cooling mode, indoor coil 32 functionsas an evaporator to transfer heat from the air flowing through coil 32on the outside thereof to the refrigerant flowing inside coil 32.Evaporator 32 is typically disposed downstream of a suitable air filter33, which is also disposed in the air flow path between return opening19 and supply opening 18 for space 16.

Air conditioning system 10 further includes a reheat heat exchanger coil38, through which heated refrigerant gas may be circulated directly fromthe system compressor. Reheat coil 38 extends over a portion (e.g., 50%)of the area of evaporator 32 and is downstream thereof. Reheat heatexchanger 38 is used to reheat air cooled and dehumidified by evaporator32 when system 10 is operated in a mechanical reheat mode in response toa demand for dehumidification of space 16. The use of a hot refrigerantgas to reheat air cooled and dehumidified by the evaporator is known inthe art and is described, for example, in U.S. Pat. No. 6,427,461, therelevant portions of which are incorporated by reference herein tofurther describe operation of reheat coil 38. A drain pan 53 is locatedbeneath coil 32 for collecting condensate runoff from coil 32.

System 10 also includes a motor-driven blower 48 of conventionalconstruction and a suitable electric drive motor 50. In a preferredembodiment, as shown, blower 48 is disposed within cabinet 12 in a space52 downstream of evaporator 32 and reheat heat exchanger 38 forreturning conditioned air to space 16 through opening 18. However,blower 48 may be disposed upstream of evaporator 32 and reheat exchanger38 in an alternate embodiment. Blower 48 is shown mounted on a heaterunit 51, whereby air returned to enclosed space 16 by way of opening 18may be conditioned by heater unit 51, if desired. Heater unit 51 may beone of several types of conventional heater units, wherein airdischarged from blower 48 passes over suitable heat exchange surfaceswhen heater unit 51 is operable. Heater unit 51 is mounted on adischarge or supply air duct part 48 a, which is in communication withopening 18. Heater unit 51 preferably includes one or more electricallyresistive heating elements, but may also include a combustible fuel(e.g., gas) heater.

A suitable sensor 54 is preferably interposed in duct part 48 a or may,depending on the configuration of system 10, be disposed in a suitableair supply duct farther downstream from and connected to duct part 48 a,but upstream of space 16. The location of sensor 54 and duct part 48 ais exemplary. Sensor 54 is preferably a temperature sensor for measuringthe temperature of the supply air.

Upstream of evaporator 32 is an enclosed space 55 within cabinet 12,through which air from space 16 may flow by way of opening 19 and spacesformed between a set of moveable louvers or dampers 56, which aremechanically linked to each other and to linkage 58, for example,connected to a suitable actuator or positioning motor 60. Positioningactuator or motor 60 is also connected via linkage 62 to a dampercomprising a second set of moveable louvers or dampers 64 forcontrolling the entry of ambient outdoor air into space 55 and dischargeinto space 16. The apparatus comprising the aforementioned sets oflouvers 56, 64 and actuator motor 60 connected thereto is also sometimesknown in the art as an “economizer”. Outdoor ambient air (represented byarrow 65) may be admitted to space 55 by way of a rain shroud 66, asuitable filter screen 68 and an enclosed space 70, which is partitionedfrom a space 72 by a suitable transverse partition 74.

A sensor 76 is disposed in space 70. In one embodiment, sensor 76 is atemperature sensor for sensing the temperature of ambient outdoor airadmitted to space 70. In an alternate embodiment, sensor 76 is ahumidity sensor instead of a temperature sensor for sensing the humidityof outdoor air admitted to space 70. A suitable humidity sensor 78 ispreferably disposed in return air space 72 or may, alternatively, bedisposed within space 16, for sensing the humidity of air in space 16. Aconventional air conditioning temperature sensor 80 (which may be athermostat) is disposed in a suitable location within space 16 forsensing and controlling the temperature of the air in space 16 asdetermined by an adjustable setpoint of sensor 80.

A suitable pressure relief damper 73 in FIG. 2, for space 72 may beprovided to minimize any pressure increase in space 16 if outdoor air isintroduced into space 55 by blower 48 for circulation to space 16. In apreferred embodiment of the invention, when outdoor air is introducedinto the system in response to a demand for dehumidification in space16, the relative proportion of outdoor air to return air is relativelyfixed (for example, 40% outdoor air to 60% return air) so that louvers56, 64 are controlled to maintain the fixed proportion of outdoor air toreturn air.

Referring also to FIG. 2, system 10 is controlled by amicrocomputer-based controller 90 in accordance with a preselectedcontrol program. Controller 90 is responsive to inputs from outdoor airsensor 76, space temperature sensor 80, space humidity sensor 78 andsupply air temperature sensor 54 for controlling the operation ofdampers 56, 64, blower 48 and heating unit 51.

Referring also to FIG. 3, in accordance with an embodiment of thepresent invention, outdoor air may be used to help dehumidify space 16if the outdoor air is suitable for dehumidification. The term “FAH” inFIG. 3 stands for “Fresh Air Heating” and refers to an operating mode ofsystem 10 wherein fresh (outdoor) air is brought into system 10 inresponse to a demand for dehumidification in space 16 and the supply airmay be heated as necessary to maintain the supply air temperature abovea predetermined threshold. If the outdoor air is cooler than the indoorair, then the humidity of the outdoor air is usually lower than thehumidity of the indoor air. In that case, the outdoor air may be used toprovide “free dehumidification” without having to operate the systemcompressor. However, the cooler outdoor air may lower the supply airtemperature below an acceptable limit, resulting in overcooling of space16. Therefore, it may be necessary to add heat to the supply air.

Controller 90 executes an iterative process as shown in FIG. 3. If freshair heating (FAH) is enabled at decision point 100 in response to ademand for dehumidification, dampers 56, 64 are opened to a preset valueat step 101 to admit outdoor air into the supply air stream in apredetermined ratio of outdoor air to return air (for example, 40%outdoor air to 60% return air). The preset value may be selected by thesystem installer or may be determined by the default setting in thesystem control program. Controller 90 then determines at decision point102 whether the supply air temperature is less than a predeterminedtemperature threshold (for example, 68° F.) in response to the inputfrom supply air temperature sensor 54.

If the supply air temperature is less than the threshold, heating unit51 (preferably, one or more electrical heating elements) is activated atstep 104 to heat the supply air. However, if the supply air temperatureis not less than the threshold, heating unit 51 is deactivated inaccordance with step 106. In either case, the process then proceeds todecision point 108. However, if FAH has not been enabled at decisionpoint 100, dampers 56, 64 are operated in accordance with the normal“economizer” mode, wherein dampers 56, 64 are controlled automatically,as represented by step 107. The process then proceeds to decision point108. If FAH is not enabled, system 10 cannot be operated to dehumidifythe space using fresh (outdoor) air.

At decision point 108, controller 90 determines whether there is ademand for heating or sensible cooling in space 16 in response to theinput from space temperature sensor 80. In response to a demand foreither heating or sensible cooling in space 16, FAH is disabled at step110 and the process returns to decision point 100. However, if there isno demand for heating or sensible cooling in space 16 at decision point108, controller 90 determines whether there is a demand for latentcooling (dehumidification) in space 16 at decision point 112. If thereis no demand for dehumidification in space 16, all “Reheat” modes,including FAH and mechanical reheat modes using the system compressor,are disabled at step 114 and the process returns to decision point 100.

If there is a demand for latent cooling in space 16 at decision point112, the process proceeds to decision point 116, where controller 90determines whether the outdoor air is suitable for dehumidification.This determination is based on input from outdoor air sensor 78, whichsenses either temperature or humidity of the outdoor air. If thetemperature or humidity, as the case may be, of the outdoor airsatisfies a predetermined condition (for example, outdoor airtemperature is 45° F. or less), the outdoor air is determined to besuitable for dehumidification and FAH is enabled (or remains enabled, ifalready enabled at step 100) at step 118. The process then returns todecision point 100.

If the outdoor air is determined to be unsuitable for dehumidificationof the space at decision point 116, FAH is disabled at step 120 and aselected mechanical reheat mode is enabled at set 122 in response to thedehumidification demand in space 16. The mechanical reheat mode involvesthe use of the system compressor to first cool and dehumidify the supplyair using the system evaporator 32 and then to reheat the dehumidifiedair by circulating heated refrigerant gas through reheat coil 38.Alternatively, the mechanical reheat mode may use a heat source otherthan reheat coil 38 such as a gas heating section or electrical heatingelements to reheat the dehumidified supply air. After the selectedmechanical reheat mode is enabled at step 122, the process returns todecision point 100.

One skilled in the art will recognize that in accordance with thepresent invention, fresh (outdoor) air may be used for dehumidifying anindoor space if the outdoor air is suitable from a temperature and/orhumidity standpoint and in the absence of a demand for heating orsensible cooling in the space. To prevent overcooling the space from theintroduction of cooler outdoor air into the supply air, the supply airtemperature is monitored and is adjusted to maintain a predeterminedsupply air temperature condition.

Although the preferred embodiment of the invention has been describedwith reference to heating the supply air as necessary to maintain thepredetermined temperature condition, the invention contemplates that insome cases the supply air may need to be cooled instead of heated. Forexample, if the suitability of the outside air for dehumidification isdetermined based on its humidity rather than on its temperature, in someclimates, the outdoor air may be warmer and drier than the indoor air,so that if outdoor air is brought in to help dehumidify an indoor space,the supply air may need to be cooled instead of heated to maintain apredetermined temperature condition.

The best mode for carrying out the invention has now been described indetail. Since changes in and modifications to the above-described bestmode may be made without departing from the nature, spirit and scope ofthe invention, the invention is not to be limited to the above-describedbest mode, but only by the appended claims and their equivalents.

1. A method of dehumidifying air in an indoor space, comprising thesteps of: sensing whether there is a demand for dehumidification in thespace; in response to said demand for dehumidification, determiningwhether outdoor air is suitable for dehumidification of the space; ifthe outdoor air is determined to be suitable, admitting outdoor air intothe space; and selectively temperature conditioning the outdoor airprior to admitting it into the space in accordance with a predeterminedtemperature condition for air supplied to the space.
 2. The method ofclaim 1 wherein said determining includes determining whether thetemperature of the outdoor air satisfies a predetermined outdoor airtemperature condition.
 3. The method of claim 1 wherein said determiningincludes determining whether the humidity of the outdoor air satisfies apredetermined outdoor air humidity condition.
 4. The method of claim 1wherein said admitting includes controlling the amount of outside airadmitted into the space.
 5. The method of claim 1 wherein saidselectively temperature conditioning includes sensing the temperature ofthe air supplied to the space and heating the air supplied to the spaceas needed to maintain said predetermined temperature condition, even inthe absence of a demand for heating in the space.
 6. The method of claim1 wherein said admitting includes introducing a predetermined amount ofoutdoor air into the air supplied to the space.
 7. The method of claim 6wherein said introducing includes maintaining a relatively fixedproportion of outdoor air in the air supplied to the space.
 8. Themethod of claim 1 further including sensing whether there is a demandfor temperature conditioning of the air in the space and in response tosaid demand for temperature conditioning, disabling said method.
 9. In asystem for conditioning air in an indoor space, a method ofdehumidifying the air in response to a demand for dehumidification inthe space, comprising the steps of: sensing whether there is a demandfor heating or cooling in the space; in the absence of a demand forheating or cooling in the space, determining whether outdoor air issuitable for dehumidification of the space; if the outdoor air isdetermined to be suitable, admitting outdoor air into the system; andselectively temperature conditioning the outdoor air prior to admittingit into the space in accordance with a predetermined temperaturecondition for air supplied to the space.
 10. The method of claim 9wherein said determining includes determining whether the temperature ofthe outdoor air satisfies a predetermined outdoor air temperaturecondition.
 11. The method of claim 9 wherein said determining includesdetermining whether the humidity of the outdoor air satisfies apredetermined outdoor air humidity condition.
 12. The method of claim 9wherein said admitting includes controlling the amount of outdoor airadmitted into the space.
 13. The method of claim 9 wherein saidselectively termperature conditioning includes sensing the temperatureof the air supplied to the space and heating the air supplied to thespace as needed to maintain said predetermined temperature condition,even in the absence of a demand for heating in the space.
 14. The methodof claim 9 wherein said admitting includes introducing a predeterminedamount of outdoor air into the air supplied to the space.
 15. The methodof claim 14 wherein said introducing includes maintaining a relativelyfixed proportion of outdoor in the air supplied to the space.
 16. In anair conditioning system for supplying conditioned air to an indoorspace, a method of dehumidifying supply air to the space, comprising thesteps of: sensing whether there is a demand for dehumidification in thespace; in response to said demand for dehumidification, determiningwhether outdoor air is suitable for dehumidification of the space; ifthe outdoor air is determined to be suitable, introducing apredetermined amount of outdoor air into the supply air; and selectivelytemperature conditioning the supply air in accordance with apredetermined temperature condition.
 17. The method of claim 16 whereinsaid determining includes determining whether the temperature of theoutdoor air satisfies a predetermined outdoor air temperature condition.18. The method of claim 16 wherein said determining includes determiningwhether the humidity of the outdoor air satisfies a predeterminedoutdoor air humidity condition.
 19. The method of claim 16 wherein saidselectively temperature conditioning includes sensing the temperature ofthe supply air and heating the supply air as needed to maintain saidpredetermined temperature condition, even in the absence of a demand forheating in the space.
 20. The method of claim 16 wherein saidintroducing includes maintaining a relatively constant proportion ofoutdoor air in the supply air, irrespective of whether saidpredetermined temperature condition is satisfied.
 21. The method ofclaim 16 further including sensing whether there is a demand fortemperature conditioning of the air in the space and in response to saiddemand for temperature conditioning, disabling said method.